The present invention relates to a system for measuring the height of circuit features on a substrate and more particularly to a non-contact system for detecting and measuring electrical conductor defects on a printed circuit board.
Inspecting manufactured articles for defects and for proper placement in larger assemblies is often a time consuming, yet necessary step in the production of complex devices. In the field of electronic components, inspection and measuring systems have relied on human inspectors using microscopes or other more sophisticated equipment.
Such systems are, of course, labor intensive and prone to human operator error, being dependent upon eyesight and judgment. Moreover, in the course of inspecting and measuring components a potential exists for damaging the articles themselves, due to physical handling thereof. In fact, in certain procedures for determining the integrity of wiring fabrication, so called destructive testing is undertaken on samples. If a sample fabricated at the same time and in the same manner as the manufactured product passes such a destructive testing procedure, it is assumed that the product itself is defect free. This assumption may be unwarranted in some cases.
A need has arisen for a procedure to inspect articles quickly, minimizing both manual labor and the possibility of damage to the articles during the inspection process, while increasing accuracy of the inspection. In particular, a non-contact procedure is preferred for inspecting and measuring the size and orientation of electronic components as well as for detecting wiring defects, if any, on a printed circuit board.
Dimensions of electronic components and thickness of wires or leads continue to decrease. Concomitantly, the criticality of positions of components and wires relative to the board or substrate on which they are mounted continues to increase. Accordingly, inspection procedures must be increasingly accurate.
It can be appreciated that even relatively small defects in wiring become a greater problem as the size of the wire decreases. In previous systems in which wiring dimensions were relatively great, small defects therein did not significantly affect performance of the overall system. With the ever decreasing size of wires used in sophisticated computing system sub-assemblies, however, relatively minor defects can result in performance degradation. Thus, defects that may not have even been detected in previous inspection systems must now be detected and corrected to ensure proper operation of the manufactured assemblies.
A system for measuring the two-dimensional size of geometrical features and/or their locations is described in "Noncontact Test System", by S. H. Campbell, et al, IBM Technical Disclosure Bulletin, Vol. 14, No. 12, May 1972. This system uses a CPU for analyzing data generated by a scanner. The scanner receives light reflected from an illuminating source.
A test system for measuring placement of circuit lines on a substrate is described in copending application Serial No. 460,702, filed Jan. 24, 1983, for "Apparatus for Automatic Optical Property Testing" and assigned to the assignee of the present application. In that system, an optical scanning head comprises two arrangements of linear diode arrays positioned adjacent one another. One diode array receives light reflected from a base plate while the other diode array receives light reflected from the surface of a conductor, provided that the latter is at the desired nominal height. Detected defects are marked and may be displayed on a TV screen.
Light has also been used heretofore to detect defects in certain goods by responding to non-uniformities in light reflectance therefrom. U.S. Pat. No. 3,712,466, issued to Aubry, et al, for example, teaches a system for optically inspecting shell casings which are conveyed successively over a predetermined path. A photocell and lens system detects reflected light from the shell and determines, by non-uniformity in the reflected light level, whether a flaw or defect exists.
U.S. Pat. No. 4,339,664, issued to Wiklund, et al, teaches a method and apparatus for topographic measurements of a charged mass in a blast furnace. A distance meter is provided near the top of the furnace for detecting direct reflection against the upper surface of the charge. The distance meter has an aiming device so that measurements of selected parts of the surface can be measured.
Devices have also been used heretofore to measure the height of an object or target relative to a surface. Such devices are often used in radar systems. Visible light may also be used to measure the distance between such a target and a surface. U.S. Pat. No. 3,669,540, issued to Rattman, et al, for example, discloses one such system in which a laser on board a helicopter is used to measure the depth of the ocean or of a submerged object. In this aforementioned reference, the altitude of the helicopter is determined by measuring the time interval between the transmission of a laser pulse and the detection of the resulting energy reflected from the ocean surface. The time interval between the transmitted laser pulse and the detected reflected energy from the surface of the ocean is compared to the time interval between the transmitted laser pulse and the detected reflected energy from the ocean floor or submerged object in order to arrive at the relative depth thereof. Such a system requires accurate time measurement, using the speed of light as a factor.
Systems have also been disclosed for use with considerably more complex light detecting apparatus. For example, U.S. Pat. No. 4,349,277, issued to Mundy, et al, discloses a parallax system for measuring and mapping a surface profile in which a light beam is split into two beams, each having a different frequency. Two images are projected onto a color pattern and a shift in the relative distance between the pattern and the light source can be detected, indicating roughness variations.
U.S. Pat. No. 4,017,188, issued to Sawatari, discloses an optical system for determining surface roughness which includes two light detectors that view two separate images of a spot focused by a lens through a slit. The respective detector signals are divided to produce signals corresponding to the ratio thereof, which in turn correspond to variations of surface height occurring as the illuminated spot is scanned across the surface.
Similarly, U.S. Pat. No. 3,713,739, issued to Zarezankov, et al, discloses a system in which a beam of light strikes each of the opposite surfaces of a rolled product, producing an image onto two separate screens of electron beam tubes. The distance between the images is measured and used to derive the distance of linear cross-sectional dimensions of the rolled product.
It would be advantageous to provide an optical non-contact system for measuring the height of circuit features on a substrate without the need for complex optical or time measurement apparatus.
It would also be advantageous to provide a system for three-dimensional optical measurement with the use of only one imaging device capable of distinguishing substrate energy reflectance from circuit feature energy reflectance.
It would further be advantageous to perform such measurement independent of light reflection time and/or substantially instantaneously.
It would also be advantageous to measure the height of such circuit features relative to a substantially planar but non-uniform substrate on which they are mounted.
It would also be advantageous to use the level of intensity of reflected light both to determine the height of circuit features and to detect and compensate for non-linearities in the upper surface of the substrate on which they are mounted.